Effects of Cu/Mg ratio on the microstructure, mechanical and corrosion properties of Al-Li-Cu-Mg-X alloys

The effects of the Cu/Mg ratio on the microstructure, properties and fracture morphology of Al-Li-Cu-Mg-X (X = Mn, Zr, Zn) alloys were investigated. The results showed that the modification of the alloy composition with different Cu/Mg ratios significantly corresponds to various microstructures, which affects the corrosion resistance, hardness and tensile properties. For instance, when the Cu/Mg ratio varies from 0.83 to 4.33, the ultimate tensile strength of the Al-Cu-Li-Mg-X alloys gradually increases from 267.2 to 366.6 MPa. Corrosion tests revealed that increasing the Cu/Mg ratio markedly enhances the corrosion resistance of the alloy. The constitution and morphology of the precipitates are also found to vary with the Cu/Mg ratio. The main precipitates are δ'(Al3Li) and the minor TB(Al7Cu4Li) phase when the Cu/Mg ratios are 1 and 0.83. By increasing the Cu/Mg ratio to 2.35, the θ'(Al2Cu) phase can be detected, while only the T1(Al2CuLi) phase is detected in the alloy with the highest Cu/Mg ratio. The combinative hardening of the T1, TB, θ' and δ' precipitates is responsible for the improvement in the mechanical properties and corrosion resistance of the alloy with the highest Cu/Mg ratio. During the tensile testing, the failure mode is converted from a typical shallow dimple fracture to an intergranular-dominated mixed one with the decrease of the Cu/Mg ratio.